It has long been recognized that the lighter weight and better heat transfer properties make aluminum alloys the logical choice as a material for internal combustion engine blocks and liners. However, most aluminum alloys lack wear resistance and it has been customary in the past to chromium-plate the cylinder bores in the engine block, or alternately, to apply cast iron liners to the cylinder bores. It is difficult to uniformly plate the cylinder bores and, as a result, plating is an expensive operation, and in the case of chromium plating, not environmentally friendly. The use of cast iron liners increases the overall cost of the engine block, as well as the weight of the engine.
Aluminum-silicon alloys containing less than about 11.6% by weight of silicon are referred to as hypoeutectic alloys, while alloys containing more than 11.6% silicon are referred to as hypereutectic alloys.
Hypoeutectic aluminum-silicon alloys have seen extensive use in the past. The unmodified alloys have a microstructure consisting of primary aluminum dendrites, with a eutectic composed of acicular silicon in an aluminum matrix. However, the hypoeutectic aluminum-silicon alloys lack wear resistance.
On the other hand, hypereutectic aluminum-silicon alloys, those containing more than about 11.6% silicon, contain primary silicon crystals which are precipitated as the alloy is cooled between the liquidus temperature and the eutectic temperature. Due to the large precipitated primary silicon crystals, these alloys have good wear resistant properties, and while alloys of this type have good fluidity, they have a relatively large or wide solidification range. The solidification range, which is a temperature range over which the alloy will solidify, is the range between the liquidus temperature and the invariant eutectic temperature. The wider the solidification range, the longer it will take for an alloy to solidify at a given rate of cooling. Thus, for casting purposes, a narrow solidification range is desired.
Typical wear resistant aluminum-silicon alloys are described in U.S. Pat. No. 4,603,665 and 4,969,428. U.S. Pat. No. 4,603,665 describes a hypereutectic aluminum-silicon casting alloy having particular use in casting engine blocks for marine engines. The alloy of that patent is composed by weight of 16% to 19% silicon, 0.4% to 0.7% magnesium, less than 0.37% copper, and the balance aluminum. The alloy has a narrow solidification range providing the alloy with excellent castability, and as the copper content is maintained at a minimum, the alloy has improved resistance to salt water corrosion.
U.S. Pat. No. 4,969,428 is directed to a hypereutectic aluminum-silicon alloy containing in excess of 20% by weight of silicon, and having an improved distribution of primary silicon in the microstructure. Due to the high silicon content of the alloy, along with the uniform distribution of primary silicon in the microstructure, improved wear resistance is achieved.
It has been recognized that as the silicon content of hypereutectic aluminum-silicon alloys is increased, the volume fraction of primary silicon particles in the microstructure will correspondingly increase, and this microstructure change will be associated with an increase in wear resistance for the alloy. However, it has also been recognized that as the silicon content of the hypereutectic aluminum-silicon alloy is increased, feeding problems, as well as floatation problems, can occur because the solidification range increases with an increased silicon content. As a result, the wear resistant properties achieved by an increased silicon content in hypereutectic aluminum-silicon alloys have been compromised, for the attainment of casting properties that allow sound castings to be produced.
Various casting techniques have been used in the past to cast alloys having a wide solidification range. One casting process, referred to as "squeeze" casting, applies pressure to the molten metal through use of a hydraulic ram, and acts to forge the "mushy" liquid and solid phases for casting soundness. However, the "squeeze" casting process is slow, and is restricted to simple shapes or configurations.
Another casting process utilized in the past for alloys having a relatively wide solidification range is centrifugal casting. Cast iron pipes and liners have been made in the past by centrifugal casting techniques, and the centrifugal casting process is capable of producing shrink-free iron pipe castings of high quality. Because the microstructure of cast iron consists of a continuous graphite phase intermingled within another continuous phase, i.e. the matrix ferrous phase, segregation of the graphite phase and the ferrous phase does not occur to any significant degree in the centrifugal casting process. As a result, centrifugal casting can produce sound iron castings by feeding the shrinkage without a modification of the distribution of the phase constituents.